This invention relates to tapered roller bearings and gear shaft support devices for vehicles.
Tapered roller bearings are suitable to support radial load, axial load and combined load. Because of their large load capacity, they are used to support gear shafts of power transmission devices such as differentials and transmissions in automobiles and construction machines.
FIG. 1 shows an automotive differential in which a gear shaft is supported by tapered roller bearings which is one of the embodiments of the present invention. It basically comprises a drive pinion 4 rotatably supported in a housing 1 by two tapered roller bearings 2, 3, a ring gear 5 meshing with the drive pinion 4, a differential gear case 7 carrying the ring gear 5 and rotatably supported in the housing 1 by a pair of tapered roller bearings 6, pinions 8 mounted in the differential gear case 7, and a pair of side gears 9 meshing with the pinions 8. These members are mounted in the housing 1 in which is sealed gear oil. The gear oil also serves as a lubricating oil for the tapered roller bearings 2, 3, 6.
FIG. 10 shows one conventional type of tapered roller bearing. It comprises an outer ring 52 having a conical raceway 51, an inner ring 56 having a conical raceway 53, a large rib surface 54 on the large-diameter side of the raceway 53 and a small rib surface 55 on its small-diameter side, a plurality of tapered rollers 57 rollably arranged between the raceway 51 of the outer ring 52 and the raceway 53 of the inner ring 56, and a retainer 58 keeping the tapered rollers 57 circumferentially spaced a predetermined distance from each other. The distance between the large rib surface 54 and the small rib surface 55 of the inner ring is designed to be slightly longer than the length of the tapered rollers 57.
The tapered rollers 57 are designed to come into line contact with the raceways 51 and 53 of the outer ring 52 and the inner ring 56 with the cone apexes of the tapered rollers 57 and the raceways 51, 53 converging on a point O on the centerline of the tapered roller bearing. By this arrangement, the tapered rollers 57 can roll along the raceways 51, 53.
With such a tapered roller bearing, the raceways 51, 53 have different cone angles, so that the combined force of loads applied to the tapered rollers 57 from the raceways 51, 53 acts in such a direction as to push the tapered rollers 57 toward the large rib surface 54 of the inner ring 56. Thus, during use of the bearing, the tapered rollers 57 are guided with their large end faces 59 pressed against the large rib surface 54, so that the large end faces 59 and the large rib surface 54 are in sliding contact with each other.
On the other hand, since the distance between the large rib surface 54 and the small rib surface 55 is designed to be slightly longer than the length of the tapered rollers 57, as shown enlarged in FIG. 11, the small rib surface 55 does not contact the small end faces 60 of the tapered rollers 57 such that small clearances exist therebetween. Also, the small rib surface 55 is formed by a surface inclined outwardly relative to the small end faces 60 of the tapered rollers 57. In the bearing manufacturing steps, the small rib surface 55 and the small end faces 60, which are kept out of contact with each other, are not finished by grinding.
In mounting such a tapered roller bearing in a mounting position, as shown in FIG. 12A, the assembly comprising the inner ring 56, the tapered rollers 57 and the retainer 58 is inserted into the raceway 51 of the outer ring 52 from above with the large end faces 59 of the tapered rollers 57 facing up. At this time, since the tapered rollers 57 have freedom relative to the inner ring 56 and the retainer 58, they will not seat in position, and their small end faces 60 are brought into contact with the small rib surface 55. This is an initial assembled state in which clearance xcex4 is present between the large end faces 59 and the large rib surface 54 of the inner ring 56.
Next, the tapered roller bearing in the initial assembled state is temporarily mounted on a mounting position of a mating device. As shown in FIG. 12B, when break-in is carried out at a low speed of about 50-100 rpm while applying an axial load Fa to the end face of the inner ring 56, the tapered rollers 57 will move a distance equal to the gap xcex4 toward the large rib surface 54, until as shown in FIG. 12C, the large end faces 59 come into contact with the large rib surface 54 of the inner ring 56, so that they settle at a regular position during use of the bearing where a gap xcex4 exists between the small end face 60 and the small rib surface 55.
Thereafter, the tapered roller bearing is preloaded axially under a predetermined load. This preloading is carried out to prevent axial movement of the tapered rollers 57 during use of the bearing, and to stably bring the tapered rollers into line contact with the raceways 51, 53 of the outer ring 52 and the inner ring 56. The control of preloading force is carried out by measuring the shaft torque, and preloading ends when the shaft torque reaches a predetermined value.
Since the power transmission device such as a differential has many gear meshing portions and sliding portions of rotary members, foreign matter such as metallic powder produced by wear at these portions can enter gear oil sealed in the housing. Such powder will penetrate into tapered roller bearings for supporting gear shafts, which are rotating under high load, thus shortening the working life of the tapered roller bearings.
Also, when such tapered roller bearings are used to support gear shafts of a differential which rotates at high speed under high load, since the large end faces of the tapered rollers are brought into sliding contact with the large rib surface of the inner ring, torque due to the slide contact increases. Further, due to frictional heat buildup, the temperature of the bearing portion will rise, thus lowering the viscosity of gear oil. This may cause shortage of oil film.
Further, in mounting the tapered roller bearing on a mounting portion, if the gap between the large end faces 59 of the tapered rollers 57 and the large rib surface 54 is large in the initial assembled state shown in FIG. 12A, break-in time tends to be long until the tapered rollers 57 settle in their regular positions shown in FIG. 12C. As shown in FIG. 11, since the small rib surface 55 of the inner ring 56 is formed inclined outwardly relative to the the small end faces 60 of the tapered rollers 57, variation in the gap between the large end faces 59 and the large rib surface 54 in the initial assembled state is large for the following reasons, and the abovementioned break-in time until all the tapered rollers 57 settle in their regular positions tends to become even longer.
Generally, the small end faces of the tapered rollers remain as forged surfaces, so that chamfer dimensions and shape are large in variation. Variations in chamfer dimension and shape are present not only between tapered rollers but in a circumferential direction of one tapered roller. As shown by solid and chain lines in FIG. 11, if the chamfer dimension and shape of the small end faces 60 differ from each other, the following will result. In the case of the small end faces 60 shown by solid line, in the initial assembled state, point P1 on the small end face 60 comes into contact with point Q1 on the small rib surface 55, so that the gap xcex4 when the tapered rollers 57 settle will be xcex41. On the other hand, in the case of the small end face 60 shown by chain line, in the initial assembled state, point P2 comes into contact with point Q2, so that the gap xcex4 when the tapered rollers 57 settle will be xcex42. Thus, due to differences in chamfer dimension and shape of the small end faces 60, the time until each tapered roller 57 settles in position tends to vary, so that longer break-in time is required.
An object of this invention is to ensure a long endurance life for a tapered roller bearing and a gear shaft support device for a vehicle.
Another object is to reduce torque loss and heat buildup due to friction.
A further object is to shorten break-in time.
According to this invention, there is provided a tapered roller bearing comprising an outer ring having a conical raceway, an inner ring having a conical raceway and formed with a large rib surface on the large diameter side of the conical raceway, a plurality of tapered rollers rollably arranged between the raceway of the outer ring and the raceway of the inner ring, and a retainer for keeping the tapered rollers circumferentially spaced a predetermined distance from each other, characterized in that the outer ring, the inner ring and the tapered rollers are all formed from a steel having an oxygen content of 9 ppm or less, and that a carbo-nitrided layer having a carbon content of 0.80 wt % or more and a Rockwell hardness HRC of 58 or more is formed on surfaces of the outer ring, the inner ring and the tapered rollers, and that the retained austenite content of the carbo-nitrided layer is 25 to 35 vol %.
The outer ring, inner ring and tapered rollers are formed from a steel having an oxygen content of 9 ppm or less in order to minimize any nonmetallic inclusions formed by oxides in the steel, improve the mechanical characteristics and fatigue properties, and to sufficiently ensure bearing life under clean lubricating oil. A steel having an oxygen content of 9 ppm or less can be obtained e.g. by a method of degassing molten steel.
Carbo-nitrided layers are formed on the surfaces of the outer ring, inner ring and tapered rollers for the following reasons. Retained austenite in a carburized layer obtained by normal carburizing has high toughness and work hardening properties. Thus a proper amount of retained austenite ensures hardness of the carburized layer and suppresses initiation and progression of cracks. But it is unstable against heat.
In contrast, if these parts are subjected to carbo-nitriding treatment under suitable conditions, nitrogen atoms will solid soluted in retained austenite, and thus serve to stabilize the retained austenite against heat and also properly keep the properties of the carbo-nitrided layer against a temperature rise due to temperature rise at the bearing portion. In a carbo-nitrided layer obtained by such carbo-nitriding treatment, a greater compressive residual stress is formed, so that it is also possible to further increase fatigue strength.
The retained austenite content should be set at 25-35 vol % to give the carbo-nitrided layer proper toughness, and to relieve excessive increase in stress due to biting of debris. If the retained austenite content is less than 25 vol %, toughness would be insufficient. If over 35 vol %, the hardness would be too low, thus resulting in deterioration in surface roughness due to plastic deformation.
The structure of such a carbo-nitrided layer as mentioned above can be formed by the following treatment steps. After heating and holding the part for a predetermined time period while keeping the carbon potential at 0.8% or over in a carburizing atmosphere, it is quenched in oil and is subjected to hardening. Thereafter it is heated and held for a predetermined time period in ammonia gas for nitriding. It is also possible to employ a method in which nitriding is carried out during carburizing. In order to adjust the retained austenite content, sub-zero treatment or tempering may be carried out.
According to this invention, a carbo-nitrided layer having a carbon content of 0.80 wt % or over and a Rockwell hardness HRC of 58 or over may be formed on the surfaces of the outer ring, inner ring and tapered rollers, the retained austenite amount of this carbo-nitrided layer being 25 to 35 vol %, and crownings may be formed at both ends of the raceway of the inner ring, the width of the crowning at each end being 20% or less of the width of the raceway of the inner ring.
The crowning is formed at each end of the raceway of the inner ring in order to prevent excessive edge loads from acting on the rollers and the raceway of the inner ring. The width of these crownings should be 20% or less of the width of the raceway of the inner ring because if it exceeds 20%, the contact surface pressure at the central portion of the raceway would be excessive.
By forming a crowning having a moderate curvature on a portion of the raceway of the inner ring except both ends at which the crownings are formed, the surface pressure distribution on the raceway can be made more uniform.
According to this invention, the small rib surface of the inner ring may be formed by a surface parallel to the small end faces of the tapered rollers, the value R/RBASE being 0.75 to 0.87, where R is the radius of curvature of the large end faces of the tapered rollers, and RBASE is the distance from the apex of the cone angle of the tapered rollers to the large rib surface of the inner ring.
The small rib surface of the inner ring is formed by a surface parallel to the small end faces of the tapered rollers for the following reasons. As shown enlarged in FIG. 6B, by forming the small rib surface 34 of the inner ring 35 from a surface parallel to the small end faces 39 of the tapered rollers 36, it is possible to minimize the influence of variations in chamfer dimension and shape of the small end faces 39 of the tapered rollers 36 against the gap between the large end faces 38 of the tapered rollers 36 and the large rib surface 33 of the inner ring 35 in the initial assembled state (which is equal to the gap between the small end faces 39 of the tapered rollers 36 and the small rib surfaces 34 of the inner ring 35 when the tapered rollers 36 have settled in position). As shown by chain line in FIG. 6B, even if the chamfer dimensions and shapes of the small end faces 39 differ, in the initial assembled state, since the mutually parallel small end faces 39 and small rib surface 34 are brought into surface contact, the gap between the large end faces 38 and the large rib surface 33 is always constant. Thus it is possible to reduce differences in time required until each tapered roller settles and thus to shorten the break-in time.
The ratio of the radius of curvature R of the large end faces of the tapered rollers to the distance Rbase from the apex of the cone angle of the tapered rollers to the large rib surface of the inner ring, R/Rbase should be set at 0.75 to 0.87 for the following reasons.
FIG. 7 shows the results of calculation using the Karna""s equation, where t is the thickness of oil film formed between the large rib surface of the inner ring and the large end faces of the tapered rollers. The ordinate shows the ratio t/to, which is the ratio to oil film thickness to when R/Rbase=0.76. The oil film thickness t is the maximum when R/Rbase=0.76, and decreases sharply when R/RBASE exceeds 0.9.
FIG. 8 shows the results of calculation for determining the maximum hertz stress p between the large rib surface of the inner ring and the large end faces of the tapered rollers. The ordinate shows, like FIG. 7, the ratio p/po, which is the ratio to maximum hertz stress po when R/Rbase=0.76. The maximum hertz stress p monotonously decreases with an increase in R/Rbase.
In order to reduce torque loss and heat buildup due to sliding friction between the large rib surface of the inner ring and the large end faces of the tapered rollers, it is desirable to increase the oil film thickness t and reduce the maximum hertz stress p. Based on the calculation results of FIGS. 7 and 8 and the below-mentioned seizure resistance test results, the present inventors determined the suitable range of R/Rbase at 0.75-0.87. For conventional tapered roller bearings, the R/Rbase value is designed at a range of 0.90-0.97.
By forming the surface roughness Ra of the large rib surface of the inner ring in the range of 0.05-0.20 xcexcm, the oil film thickness t between the large rib surface of inner ring and the large end faces of the tapered rollers, and the lubricating condition between these surfaces can be maintained in a proper state.
The surface roughness Ra should be 0.05 xcexcm or over for the following reasons. As shown in FIG. 12B, when the tapered roller bearing is mounted, break-in is carried out at a low speed of 50-100 rpm while applying an axial load Fa to the end face of the inner ring 56. If the surface roughness Ra is less than 0.05 xcexcm, the lubricating state between the large rib surface 54 of the inner ring 56 and the large end faces 59 of the tapered rollers 57 will involve a mixture of fluid lubrication and boundary lubrication during break-in, so that the friction coefficient varies considerably and the measured shaft torque varies widely. This worsens the preload control accuracy. If Ra is 0.05 xcexcm or over, the lubricating state will be boundary lubrication, so that the friction coefficient stabilizes and thus preload control is possible with high accuracy. Under normal bearing use conditions where speed exceeds 100 rpm, sufficient oil film is formed between the large rib surface 54 and the large end faces 59, so that the lubricating state between these surfaces becomes fluid lubrication, and the friction coefficient decreases.
The surface roughness Ra should be 0.20 xcexcm or under because if Ra is over 0.20 xcexcm, the temperature will rise at the bearing portion in the high-speed rotation region, so that when the viscosity of lubricating oil decreases, the oil film thickness tends to be insufficient and seizure tends to occur.
By restricting the gap xcex4 formed between the small rib surface of the inner ring and the small end faces of the tapered rollers when the large end faces of the tapered rollers are in contact with the large rib surface of the inner ring to not more than 0.4 mm, it is possible to reduce the number of revolutions required for the tapered rollers to settle in position during the break-in, and to shorten the break-in time. The permissible maximum value of the gap xcex4, that is, 0.4 mm, was determined based on the results of the below-described break-in test.
By forming the small rib surface of the inner ring by grinding or turning, it is possible to accurately control the gap between the small rib surface of the inner ring and the small end faces of the tapered rollers.
The tapered roller bearing of this invention may have the large rib surface of the inner ring made up of a conical surface in contact with the large end faces of the tapered rollers, and a flank smoothly connecting with the conical surface and curving in a direction away from the large end faces of the tapered rollers.
By smoothly connecting the curved flank to the conical surface of the large rib surface of inner ring in contact with the large end faces of the tapered rollers and forming an acute-angle, wedge-shaped gap near the outer edge of the contact region, it is possible to increase the function of drawing lubricating oil into the contact region and to form a good oil film. Also, by the formation of the smooth flank, it is possible to prevent damage due to abutment with the large rib surface of inner ring when the tapered roller skews.
By employing an arc as the sectional shape of the flank, it is possible to easily form a flank that is superior in the lubricating oil drawing function.
By providing a circular recess on the central portion of the large end faces of the tapered rollers, and extending the outer peripheral end of the recess to near the boundary between the conical surface and the flank of the large rib surface of the inner ring, it is possible to guide lubricating oil to near the wedge-shaped gap and to supply a sufficient amount of lubricating oil into the wedge-shaped gap, and also to further increase the permissible skew angle of the tapered rollers.
By providing the boundary between the conical surface and the flank of the large rib surface of inner ring near the outer edge of the maximum contact oval produced by the contact between the large end faces of the tapered rollers and the large rib surface of the inner ring under the maximum permissible axial load of the tapered roller bearing, it is possible to suitably form the wedge-shaped gap for drawing the lubricating oil in the entire load range of the tapered roller bearing.
Also, in this invention, in a gear shaft support device for a vehicle in which a gear shaft is rotatably supported by a tapered roller bearing in a housing in which is sealed gear oil, the outer ring, inner ring and tapered rollers of the tapered roller bearings are formed from a steel having an oxygen content of 9 ppm or less, and a carbo-nitrided layer having a carbon content of 0.80 wt % or more and a Rockwell hardness HRC of 58 or more is formed on each of their surfaces, the carbo-nitrided layer having a retained austenite amount of 25 to 35 vol %. Thus it is possible to markedly prolong the maintenance cycle of differentials and transmissions, etc.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: